Marcu 4, 1915 | 
NATURE 19 

shades for which have also been hitherto mainly 
imported from abroad. ‘This particular kind of glass 
is of pressing importance in relation to coal-mining, 
and it is certainly humiliating to learn from the 
makers of these lamps that for glass of the required 
quality capable of complying with the Home Office 
tests, we have been so largely dependent upon foreign 
glass manufacturers. . 
With respect to the supplies of chemical reagents, 
the joint committee found it necessary to entrust to 
a special sub-committee the somewhat arduous task 
of compiling a list of all the commonly used reagents 
with indications of the standard of purity required 
and the tests necessary for ascertaining whether the 
required standard had been reached. The list has been 
published as a pamphlet, and has been sent to many 
firms and companies of manufacturing chemists with 
the view of ascertaining which reagents of their own 
manufacture they are prepared to supply. When the 
replies have been received the joint committee will 
know the requirements of the profession could be met 
by British manufacturers. 
It is hoped that sooner or later laboratory supplies 
both of apparatus and materials will be entirely of 
British origin. The manufacturers are—in some cases 
at considerable cost—developing lines of industry 
which are of the nature of new departures for this 
country. It is the duty of consumers and users—in 
fact, of every branch of the profession—to do their 
utmost to encourage and support these new home 
industries. Patriotism and the credit of our country 
alike demand that, after the war, they should help 
those who are helping them by insisting upon having 
nothing but the products of British manufacture. 
They should not only assist in the development of these 
industries now, but insure their permanent retention 
after the declaration of peace. With the achievement 
of this result there would be removed the reproach that 
the nation which gave to chemical science Priestley, 
Black, Boyle, Cavendish, Davy, Dalton, Faraday, and 
Graham—the country which founded the coal-tar 
colour industry, and which had taken the lead in the 
manufacture of “heavy chemicals,’’ allowed her labora- 
tory work to be dependent upon foreign materials, and 
her great textile and metallurgical industries to be 
threatened through the stoppage of supplies from 
inimical countries. 

LORD KELVIN’S WORK 
STADIES.S 
V.—Gyrostatic Theory of Elasticity. 
(Woze.--In the explanation of steady precession, near the foot of the first 
column of p. 715 of Nature of February 25, the words, ‘‘ the horizontal axis 
A of the couple,” referred to a cut, which, owing to an accident, could not 
be given. But Fig. 6 there printed, and repeated here on page ar. will serve 
instead. In that, as indicated in the small diagram at the bottom of the 
figure, the axis of angular momentum—the spin-axis—is to be supposed 
drawn towards the right, from the centre of the gyrostat along the (hori- 
zontal) axis of rotation, and the axis of the couple horizontally from the 
centre towards the observer. The dotted arc, marked 90°, should be 
continued round to the axis marked m2w. The angle go’ is that between 
the sp n-axis and the couple-axis.—A.G.] 
Oe other experiment I shall make with the veteran 
gyrostat, which has been spun again. You see 
that the rim carries two trunnions in line with the 
centre of the wheel (Fig. 10). These are placed on 
bearings attached to this square wooden frame; and 
now you see that as I hold the tray in my hands in a 
horizontal position, the gyrostat rests with its axis 
vertical or nearly so. The direction in which the wheel 
is spinning is shown by the arrow on the upper side. 
I now carry the tray round in azimuth in the direction 
of spin: nothing happens; the gyrostat spins on 
8 Abriiged from the Sixth Kelvin Lecture, delivered atthe Institution of 
Electricel Engineers, on January 28, by Prof. A. Gray, F.R.S. (Continued 
from Nature, No. 236s, vol. xciv.,,p. 716.) 
NO. 2366, VOL. 95] 
ON GYRO- 
ja very decided will of its own. 

placidly. lf, however, I carry the tray slowly round 
the other way, the gyrostat immediately turns upside 
down on the trunnions; and now, as I go on carrying 
the tray round in the same direction as before, the 
gyrostat is quiescent as at first; but the spin, by 
| the inversion of the gyrostat, has been brought into 
the same direction as the azimuthal motion. 
The gyrostat behaves as if it possessed volition— 
It cannot bear to be 
carried round in the direction opposed to the rotation, 
and, as it cannot help the carrying round, it accom- 
modates itself to circumstances by inverting itself so 
that the two turning motions are made to agree in 
direction. Again I reverse the azimuthal motion, and 
the gyrostat inverts itself so that the wheel turns in 
the same direction in space as at first. 
The inversion brings into play a wrench on the 
hands of the experimenter. A varying couple, lasting 
during the time of the inversion, is required to reverse 
the angular momentum of the wheel in space, and 
this is applied to the gyrostat by the frame at the 
trunnions, and to the frame, because that is kept 
steady, by the hands of the operator. The _ total 
change of angular momentum is 2 N, where N is the 
angular momentum of the flywheel, and this is the 
time-integral of the couple. 
It will be noticed that in this experiment, in which 
the gyrostat displays this curious one-sided stability 
and instability, it is affected by a precession im- 
pressed upon it from without. The system was not 

left to itself, I carried it round. The gyrostat had 
little or no gravitational stability—the centre of 
gravity was nearly on a level with the trunnions; 
but even if it were gravitationally unstable, suffi- 
ciently rapid azimuthal motion would keep it upright 
if that motion agreed with the spin, while the least 
motion the other way round would cause it to capsize. 
It is important to notice that if the gyrostat be 
placed on the trunnions, so that the axis of the wheel 
is in the plane of the frame, azimuthal turning in 
one direction causes one end of the axis to rise, or 
turning in the other direction causes the other end 
to rise. As I shall show presently, this means a 
reaction couple on the frame which must be balanced 
by a couple applied by the experimenter. 
“Better than anything else I know, this experiment 
of the capsizing of the gyrostat by azimuthal motion 
affords an example of the two forms of solution of a 
certain differential equation, which, when the gyrostat 
is without sensible gravitational stability, and 6 is 
small, I may write 
a6 
A 
at’ 
+wN6=0, 
where N is the angular momentum of the wheel, and 
w» the angular speed with which the tray was carried 
round. When the turnings were in the same direc- 
tion, » and N had the same sign, but when the 
turnings were in opposite directions the product w N 
had a negative value. When the product. is positive 
